This invention relates to a telescopic data coupler for data and/or energy transmission between mechanical parts that are coaxial and in relatively close engagement, but not fixed relative to each other, such as telescopic joints, hydraulic rams and the like. It is particularly suitable for use in hostile environments such as oil wells, process plant or underwater, and where unlimited rotational freedom between the parts is required.
One specific application is in data transmission between parts of so-called Measurement While Drilling systems (MWD) that are in common use in earth drilling applications. The function of an MWD system is to measure and record parameters of the borehole, the surrounding earth formation, drillstring or the drilling operation itself and to transmit some or all of the data gathered from such measurements to the earth's surface as the drilling operation continues. This application will be referred to in the following description without implying any limitation on the scope of the invention being implied.
MWD systems are often designed to be installed concentrically in drilling tubulars, which are thick walled steel tubes interconnected by screw threads to make up a complete drill string. Typically such tubulars are 1-10 m long and vary in outer diameter from 50 mm up to 250 mm. The instruments and electronics of the MWD system are themselves typically contained in tubular pressure housings concentrically installed in the drill string. The outside diameter of the MWD tubulars is significantly less than the inside diameter of the drilling tubulars, leaving an annular space for the passage of drilling fluid.
The MWD tubulars usually require to be interconnected electrically for the passage of data and sometimes also of power, while remaining protected from the high pressure, slurry-like drilling fluids and high vibration levels in the environment. Inter-connectors for this purpose provide mechanical and electrical engagement together with hydraulic seals against the high pressure drilling fluid by which they are surrounded when in service.
Where an MWD tubular joint is contained wholly within a drilling tubular, it is a relatively simple matter to make a secure and well-protected electrical connection, sealed against the ingress of high pressure fluids, between the parts concerned. The MWD tool is assembled independently and then loaded as a single item into the drilling tubular.
When the MWD system crosses the drill string joints, two cases arise. If the MWD assembly is fixed only to a single tubular then there are no specific constraints on the length of the part that resides inside the adjacent parts of the drillstring. But if the two or more parts of the MWD assembly must also be secured rigidly each to its own tubular then it becomes necessary to match closely the lengths of the inner and outer tubulars. This situation arises frequently for example when one tubular contains a transmitter to send data to the surface and another contains a measuring instrument. Because the two or more parts of the drillstring are connected by screw threads, the inner inter-connector must provide freedom for relative rotation of its two parts in addition to having the properties mentioned above.
Drill string components, and particularly their threaded ends, are vulnerable to damage during drilling operations. It is normal practice to recut the threads on such parts many times during their working life. In MWD operations the outer tubulars may be specially machined to house the MWD elements or made from special alloys. Consequently they are expensive and it would be highly uneconomic to scrap them merely because a threaded joint was damaged. This means that the inner tubulars of the MWD system must be capable of being varied in length to ensure that they can be fitted in the available outer tubulars.
Accordingly it is convenient to provide an adjustable-length, rotatable connection between the inner, MWD, tubulars. Conventional means include spring-loaded coaxial multi-conductor connectors, but these typically have only a very small adjustable range and are relatively expensive.